Recently, in the photolithography technique, with the trend toward a higher degree of integration and a higher function of an integrated circuit, the refinement of the integrated circuit is advancing. The exposure tool is hence required to form a circuit pattern image with high resolution on a wafer surface at a long focal depth, and shortening of the wavelength of an exposure light source is being advanced. The exposure light source is further advancing from conventional g-line (wavelength: 436 nm), i-line (wavelength: 365 nm) and a KrF excimer laser (wavelength: 248 nm), and an ArF excimer layer (wavelength: 193 nm) is coming to be employed. In the lithography technique using such light having a wavelength of from 193 to 436 nm, semiconductor devices having a circuit size of 32 to 45 nm at best can be only manufactured.
Under the foregoing technical trends, a lithography technique typically using, as an exposure light source, light having a wavelength of 13 nm among EUV lights (extreme ultraviolet lights) is considered to be applicable for semiconductor devises over generations of a circuit size of 32 nm and thereafter, and is attracting attention. The principle of image formation in the EUV lithography (hereinafter abbreviated as “EUVL”) is identical with that of the conventional lithography from the viewpoint that a mask pattern is transferred using a projection optical system. However, since there is no material capable of transmitting light therethrough in the EUV light energy region, a refractive optical system cannot be used. Accordingly, the optical systems are all reflecting optical systems.
The optical member for an exposure tool for use in EUVL is basically configured with (1) a substrate, (2) a reflective multilayer formed on the substrate, and (3) an absorber layer formed on the reflective multilayer. Since the optical member for an exposure tool for use in EUVL is reflecting type one, it is not always necessary for the substrate to have a light transmitting property. However, an extremely low thermal expansion material having transparency has been desired, in order to enable evaluation and inspection, for example, for evaluating homogeneity and surface smoothness using a interferometer or the like so as not to generate a strain even under irradiation with EUV light, or for judging the presence of internal defects such as bubbles and striae by inspection with a microscope or visual inspection.
Moreover, transparent low thermal expansion materials have been widely used in various materials for which a low thermal expansibility and transparency are strictly required, for example, materials for optical parts, materials for large mirror reflectors, materials for ring laser gyroscopes, materials for precision parts such as verification standards for precise measurement, and various electronic materials.
As the extremely low thermal expansion materials having transparency, there may be mentioned TiO2-containing silica glasses (hereinafter referred to as “TiO2—SiO2 glass”) represented by ULE #7972 (product name) of Corning Incorporated and transparent glass-ceramics represented by ZERODUR (product name) of SCHOTT. U.S. Patent Applications disclose methods in which a TiO2—SiO2 porous glass body is formed and converted into a glass body and then a mask substrate is obtained (e.g., see Patent Document 1).
The TiO2—SiO2 glass is known as an extremely low thermal expansion material having a thermal expansion coefficient lower than that of quartz glass. Also, since the thermal expansion coefficient can be controlled by the TiO2 content in the glass, a zero-expansion glass whose thermal expansion coefficient is close to 0 can be obtained. Accordingly, the TiO2—SiO2 glass involves a possibility as a material to be used in an optical member for an exposure tool for EUVL. However, in the TiO2—SiO2 glass, the temperature region where the thermal expansion coefficient is substantially zero is only limited to around room temperature. Moreover, there is an absorption resulting from Ti3+ at around 500 nm and thus the glass has a coloring property. In addition, since the glass contains a large amount of OH groups, there are absorptions at several wavelengths such as around 2700 nm.
Moreover, in the materials for optical parts, the materials for precision parts such as verification standards for precise measurement, and various electronic materials as well as optical members for exposure tools for EUVL, the temperature region where the thermal expansion coefficient is substantially zero is preferably wide. However, in the conventional TiO2—SiO2 glasses, the temperature region where the thermal expansion coefficient is substantially zero is only limited to around room temperature. Also, in the conventional glass-ceramics, since a dimensional change with a change in temperature shows hysteresis owing to structural relaxation, there is a problem in absolute dimensional accuracy and also there is a problem that a smooth surface is hardly obtained.
To the contrary, there is disclosed a TiO2-containing silica glass which is excellent in transparency and whose thermal expansion coefficient is substantially zero in a wide temperature range and a process for producing the same (e.g., see Patent Document 2). Also, there is disclosed an F-containing TiO2—SiO2 glass having a zero-expansion property in a wider temperature region and a process for producing the same (e.g., see Patent Document 3).    [Patent Document 1] U.S. Patent Application Laid-Open No. 2002/157421    [Patent Document 2] JP-T-2008-505043    [Patent Document 3] JP-A-2005-104820